Perfluorinated (PFC) gases, such as CF4, C2F6, NF3 and SF6, are commonly supplied to process chambers used in the semiconductor and flat panel display manufacturing industry for, for example, dielectric layer etching and/or chamber cleaning purposes. Following the manufacturing or cleaning process, there is typically a residual amount of the gas supplied to the process chamber contained in the gas exhausted from the process chamber. The perfluorinated compounds mentioned above are known to be greenhouse gases, and so it is desirable to remove these species from the exhausted gas prior to venting the gas into the atmosphere.
EP-A-0 694 735 describes gas abatement apparatus for treating a gas stream to remove noxious substances from a gas stream, in which a fuel gas is pre-mixed with the gas stream before it is injected through a nozzle into a combustion zone that is laterally surrounded by the exit surface of a cylindrical, inwardly-fired foraminous gas burner. A fuel gas and air are simultaneously supplied to a plenum surrounding the foraminous burner to effect flameless combustion at the exit surface, with the amount of air passing through the foraminous burner being sufficient to consume not only the fuel supplied to the burner but also all of the combustibles in the mixture injected into the combustion zone. The bottom open end of the combustion zone is connected to a cooling column having an inner surface which is coated with a stream of water to cool the gas stream leaving the combustion zone. The gas stream is subsequently separated from the cooling water and passed through a scrubber before being vented to the atmosphere.
Premixing the gas stream with a fuel gas prior to the entry of the stream into the combustion zone was found to improve the PFC abatement efficiency of the apparatus. Whilst good results were obtained with C2F6, SF6 and NF3, the technique was not applicable to the abatement of CF4 due to the maximum temperature that was attainable within the combustion zone.
A modification of the above technique is described in EP-A-0 802 370, in which the pre-mixed fuel and gas stream is injected into the combustion zone through a nozzle that is concentric with a lance that introduces oxygen into the mixture before it enters the combustion zone. Using this technique, good results were achieved for all PFC gases, including CF4. A further modification is described in WO-A-2006/013355, in which the nozzle is also surrounded by a sleeve for enabling a fuel gas to be injected into the combustion zone with the gas stream, as opposed to pre-mixing the gas stream with fuel. By varying the nature of the gases that are supplied to both the lance and the sleeve, a range of noxious substances can be treated using a single inject stoichiometry. This configuration has been found to be particularly effective at treating a fluorine (F2)-containing gas stream without the generation of CF4 as a combustion by-product.
The cost of ownership of such apparatus is dependent, amongst others, on the amount of fuel gas supplied to the foraminous gas burner. One technique which has been used to reduce fuel consumption has been to reduce the length of the foraminous burner, and thus reduce both the volume of the plenum surrounding the burner, and the quantities of fuel gas and air that need to be supplied to the plenum to effect flameless combustion at the exit surface of the burner.
The exit surface of the foraminous burner emits infrared radiation which assists in maintaining a high temperature within the combustion zone. However, relatively cool conditions prevail towards the bottom of the foraminous burner due to reduced radiation exchange. As the length of the burner is decreased, the proportion of the burner at which these relatively cool conditions prevail is increased. It has been observed that when the aspect ratio (length/internal diameter) of the burner is decreased below a value of 1, the amount of CO and non-combusted fuel gas within the gas stream exhausted from the apparatus starts to increase, and the abatement performance of the apparatus starts to decrease. This poor performance has been attributed to the increased proportion of the burner that operates at a relatively low temperature, effectively placing a limit on the extent to which the aspect ratio of the foraminous burner may be reduced.
Another factor which has affected the cost of ownership of the gas abatement apparatus has been the increase in the size of semiconductor and flat panel process chambers. There is a trend in the manufacture of such devices to conduct processing on increasingly larger substrates to deliver economies of scale, with the substrate being diced upon completion of the processing steps to produce a multiplicity of individual devices of the required size. As a result, the size of the process chambers and the flow rates of the gases supplied thereto, and subsequently exhausted therefrom, have also increased to accommodate the larger substrates and produce acceptable processing rates.
The increase in the amount of gas entering the gas abatement apparatus may be accommodated by increasing both the number of inlets through which the exhaust gas is injected into the combustion zone, and the volume capacity of the combustion zone. For the reasons discussed above, the increase in the volume capacity of the combustion zone cannot be realised by increasing the internal diameter of the foraminous burner alone (in order to accommodate the increased number of inlets required by the increased flow of exhaust gas) without detriment to the performance of the abatement apparatus. Consequently, the length of the combustion zone, and thus also both the length of the foraminous burner and the volume of the plenum surrounding the burner, must also be increased when the internal diameter of the burner is increased, thereby increasing the fuel gas consumption of the apparatus.